In recent years, automobiles of a hybrid type and of an electric type have been prevalent in terms of energy saving, with the hybridization of construction machines also being accelerated. Construction machines such as hydraulic excavators driven by a hydraulic system typically include a hydraulic pump enabling the machine to work at a maximum load, a hydraulic work implement driven by a hydraulic fluid delivered from the hydraulic pump, and an engine for driving the hydraulic pump, to meet all working requirements from light-duty to heavy-duty operations.
Heavy-duty operations, including the heavy-duty excavation involving the use of the construction machine's hydraulic work implement to excavate soil and load a truck with the soil, however, are just part of all necessary work assigned to the machine. Light-duty operations, such as leveling out the ground surface, will fail to fully utilize the performance of the engine. This forms one of the causative factors rendering it difficult to reduce fuel consumption in the hydraulic excavator. Hybrid construction machines, intended to compensate for this drawback, are known to assist part of engine power output with an electricity storage device and an electric motor combined to reduce fuel consumption. The electricity storage device mounted on such a hybrid construction machine uses a lithium ion battery, a capacitor (such as an electric double-layer capacitor or a lithium ion capacitor), or a nickel hydrogen battery, for example.
Electricity storage devices are known to commonly have a characteristic of having increases in internal resistance due to charge/discharge of current or time varying deterioration associated with storage of electrical energy (hereinafter, the internal resistance may be referred to simply as resistance). These increases in internal resistance usually irreversibly increase resistance (hereinafter, this resistance may be referred to as irreversibly increasing resistance). Some researches state, on the other hand, that only if the charge/discharge of a too large current for a particular capacity of the electricity storage device is continued, this causes reversible increases in resistance (hereinafter, this resistance may be referred to as reversibly increasing resistance), and these increases in resistance differ from usual ones. Accordingly, Patent Documents 1 and 2 disclose control schemes for detecting a reversible increase in resistance and controlling the progress of deterioration.
Patent Document 1 discloses the control scheme for detecting the reversible increase in resistance, based upon a ratio between discharge resistance of an electricity storage device and charge resistance thereof, and pausing the charge/discharge of the electricity storage device after the ratio between the discharge resistance and charge resistance of the electricity storage device has decreased to a previously set value or lower.
Patent Document 2 discloses the control scheme for detecting the reversible increase in resistance, based upon an electric current estimation error between an estimated current value and a measured current value, and upon a difference in a concentration of a battery electrolyte between the electrodes. The estimated current value is used after being estimated from a simplified modeling expression relating to electrochemical reactions within electrodes. The control scheme then limits an amount of the current, an amount of electric power, and a voltage, if a degree of the reversible increase has exceeded a previously set value.